This invention relates to contaminant detection in paper pulp and particularly to the means to detect and measure the undesirable particles in virgin pulp and waste paper pulp including sticky contaminant particles in waste paper pulp for monitoring and controlling such particle inclusion in the pulp manufacturing process.
In the manufacture of paper products, an appropriate fiber pulp is used which may be purchased in solid pulp board form or directly processed in a liquid form. With recent development in the area of the environmental impact of processes, the increasing cost of energy and the like, the paper making industry has developed the recyling of waste paper into pulp for reconstitution into paper products.
It has long been known that the resulting paper product is of course greatly affected by the inclusion of foreign matter in the basic pulp from the which the paper product is made. Dirt, pitch, particles of bark, identified in the art as shives, and the like are the usual particles encountered in virgin pulp. Various means have been provided for detecting of the foreign contaminants during the forming of the pulp product and to modify the pulp forming process to minimize the inclusion of such foreign product.
Various systems have been suggested for monitoring the presence of such particles. Generally in the prior art, the pulp is sampled and the sample manually processed to detect the particle characteristic. The patent art discloses photodetection monitoring means for particles detection of pulp. For example, U.S. Pat. No. 4,402,604 which issued Sept. 6, 1983 and U.S. Pat. No. 4,225,385 which issued Sept. 30, 1980 disclose withdrawing of a sample of the pulp stock passing of the sample through a photoelectric detection system which detects the presents of dirt, shives and the like. U.S. Pat. No. 4,402,604 which issued Sept. 6, 1983 discloses a system whereby the liquid pulp or the pulp to be inspected is passed in a liquid state between a pair of transfer plates. The pulp is mixed with a pure liquid, that is, one without fiber content to control the consistency of the mixture as it passes through the plates. Light sources are applied to opposite sides of the moving sheet and a plurality of photosensitive devices are located in a row across the pipe to receive both reflected light and transmitted light. A series of photoelectric diodes are mounted across the plate to continuously scan the moving pulp. The output of each device is a signal dependent on the intensity of the light, and therefore the presence or absence of the particles. U.S. Pat. No. 4,225,385 discloses a method for directly passing of the pulp through a mixing unit for dilution of water and then through a inspection tube having a photoelectric device to scan the liquid as it passes through. Other patents have further considered analysis of the virgin pulp for foreign matter; for example, U.S. Pat. Nos. 4,184,204, 3,709,615, 4,318,180, 4,276,119, 4,441,960. The literature has also of course discussed the practical arts of foreign material detection.
Waste paper recycling further presents particularly unique problems in connection with foreign matter. Waste paper may originate from products including sticky contaminants. For example, the waste paper may include labels, tapes, and the like having an adhesive backing, as well as coated products including various adhesive material such as waxes, asphalt, polyethylene and other plastics. Such adhesive type materials create particularly unique problems not only with respect to the final product quality but serious difficulties in connection with the operating of the paper processing machinery. Thus, the "sticky" contaminants may agglomerate within the pulp or stock and result in a poor appearance of the final product, as well as adversly affect the end use properties required for certain application, such as printers and the like. In addition because of the adhesive characteristic, sticky contaminants are particularly prone to form deposits within the process machinery, impairing the runability and efficiency of the process machinery. Various methods have been suggested and used in connection with the processing of recycling of waste paper products. Generally, the techniques will include presorting of the paper to be recycled in an attempt to remove sticky material prior to pulping and processing. An alternative method is to treat the waste paper and/or the pulp so as to render the sticky contaminants innoxious, as by dispersion or treating for easy removal. This involves modifying the manufacture and/or formulation of the adhesives and other sticky contaminants to render them subject to such processing. Finally, a standard procedure used in paper pulping and making is to monitoring the contaminants and operate cleaning and removal devices in the pulping process.
The Institute of Paper Chemistry of Appleton, Wis. has been active in the development of appropriate detection systems. For example, a paper entitled "Detection and Quantification of Sticky Contaminants and Recycled Fiber Systems" by Doshi, Dyer and Krueger setsforth the difficulties encountered in recycling of paper because of sticky contaminants and as methods developed for detection and treating of such sticky contaminants. An article "Removing Stickies from Recycled Fiber" by W. C. Krueger and D. F. Bowers, both associated with the Institute of Paper Chemistry, appears in the July 1981 of "Tappi", Vol. 64 #7, Page 39-41. A further paper on the subject is published by J. M. Voith GMBH of Heidenheim, Germany, entitled "Process Technology Solution for the Separation of the Sticky Impurities" in the 1983 issue of "Pulping". All of the papers indicate the great difficulty which has been encountered in the removal of sticky contaminants and the difficulty of monitoring the presence of such materials.
Thus, the paper by Doshi et al is a somewhat typical approach presently used in the art. In such processes, the sample of the pulp either directly or after concentration of the contaminants in a sample is formed into a hand sheet and allowed to dry for handling. The air dryed hand sheet is placed between appropriate filter paper and hot pressed to form a sheet in which the contaminants can be monitored. The hand sheet is examined under ultraviolet light in accordance with the method developed by the authors. The authors found that the hot pressing method made invisible stickies visible and tended to increase or magnify the smaller sticky contaminants. The alternative generally is used in the art is a pure visual inspection of samples as to the size and number to the dirt particles. All such processes is obviously quite subjective, time consuming and provides for correction only after a long delay of the actual cycling of the apparatus.
There is therefore a very significant demand in the art for a method and means to detect and measure, and preferably control contaminant particles, particularly of sticky contaminants, in pulp preparation for the paper manufacture industry. The system should of course preferably provide an on-line measurement which produces a quantified output of dirt-type substances in the pulp preparation, and should permit both rapid analysis and creation of appropriate control signals to permit essentially on-line corrective action within the process.